1. Field of the Invention
The present invention relates to an electric double layer capacitor, particularly an electric double layer capacitor excellent in the operation reliability.
2. Discussion of Background
The electric double layer capacitor is based on a principle to store electric charge in an electric double layer formed on the surface of an electrode impregnated with an electrolytic solution. In order to obtain a high capacitance density, it is common to employ as a material for the electrode, a carbonaceous material such as activated carbon, carbon black or fine particles of a metal or conductive metal oxide, having a high specific surface area. For efficient charging and discharging, the electrode is bonded to an electro-conductive foil or layer having low resistance, such as a metal or graphite which is called a current collector. For the current collector, it is common to employ a valve metal such as aluminum or a stainless steel such as SUS304 or SUS316L, which is electrochemically highly corrosion resistant.
As an electrolyte for the electric double layer capacitor, an organic electrolytic solution and an aqueous electrolytic solution are available. However, an attention has been drawn to an electric double layer capacitor employing an organic electrolytic solution, as the operating voltage is high, and the energy density in the charged state can be made high. In the case of an electric double layer capacitor employing an organic electrolytic solution, if water is present in the interior of an electric double layer capacitor, the water is electrolyzed and deteriorates the performance. Accordingly, the electrode is required to have water completely removed, and usually it is dried by heating under reduced pressure.
As the electrode, activated carbon is mainly used. However, activated carbon is usually in a powder form. Accordingly, it is common to use an electrode assembly which is made by firstly molding carbon into a sheet form by using a binder containing a fluorine-containing resin such as polytetrafluoroethylene (hereinafter referred to as PTFE), and then electrically connecting it to a current collector. It has been proposed to intimately contact and bond the electrode and the current collector via a conductive adhesive layer so as to minimize the electric contact resistance between the two. However, a fluorine-containing resin has a nature difficult to bond, and thus it is difficult to raise the bond strength.
The conductive adhesive layer is required to be electrochemically corrosion resistant. Therefore, as a filler which provides an electric conductivity for the conductive adhesive layer, a carbon material such as carbon black or graphite is mainly used. Further, in order to secure the bond strength, various binder components for the conductive adhesive layer have been studied, and resins such as cellulose and polyvinyl alcohol (JP-A-59-3915, JP-A-62-200715) or inorganic binder components such as water glass have been known.
However, these resins are not sufficiently resistant against an organic electrolytic solution, and if a conductive adhesive containing such a resin as a binder is used, the electrode and the current collector may likely undergo peeling. Further, the heat resistance of such resins is about 150.degree. C. at highest, whereby drying treatment by heating can not be applied after the electrode and the current collector are bonded. Therefore, the remaining water adsorbed on the activated carbon can not be completely removed, and there has been a problem such that the performance of the electric double layer capacitor tends to be poor due to the electrolysis of the remaining water.
The inorganic binder such as water glass is highly heat resistant, but it has had a problem that the bond strength of the electrode and the metal current collector tends to be insufficient, or the performance of the electric double layer capacitor tends to deteriorate due to the elution of alkali components or the remaining water.